Can a Vertex Group be saved as a separate .blend File?
The best way I can think of is, for safety duplicate
the original object first as you suggested,
then remove the other parts of the mesh:


Select the part of the mesh to keep.
Press CTRL + I to Invert Selection.
Press X and Vertices to delete excess parts.




Or (not so good solution) you go back to object mode,
select the object if it is not already the only selection,
and export (for example as collada), and check only selected.
Later you can import the exported mesh.

Or you may give a nice name, like APPEND-WITH-ME, and you can
simply append your other blend file and select that object
(not the mesh, the object). Every material, particles,
etc will be also loaded into the new blend file.